1. Field of the Invention
The subject invention is directed to systems and methods for wastewater treatment, and more particularly to systems and methods for using dissolved gas injection to improve membrane bioreactor (MBR) operations. Aspects of the present invention enable the production of high-quality effluent from a wastewater influent at a reasonable cost and over a wide range of influent and pollutant loading rates.
2. Background of the Invention
With the current focus on water reuse projects and the role they play in the water cycle, the search for cost competitive advanced wastewater treatment technologies has never before been so important. Wastewater represents a water resource and its reuse can significantly reduce the demand for water supply.
A membrane bioreactor (MBR) is an effective treatment process for water reuse and reclamation. MBR systems are no longer viewed as a novel process and are used more and more in wastewater treatment applications all over the world. A membrane bioreactor is the combination of a membrane process like microfiltration or ultrafiltration with a suspended growth bioreactor, and is now widely used for municipal and industrial wastewater treatment with plant sizes up to 80,000 population equivalent (i.e. 48 MLD).
Membrane bioreactor systems have two basic configurations: (1) the integrated bioreactor that uses membranes immersed in the bioreactor, shown in FIG. 1(a), and (2) the recirculation MBR, shown in FIG. 1(b), in which the liquor circulates through a membrane module situated outside the bioreactor.
In the integrated MBR system, the key component is the microfiltration membrane that is immersed directly into the activated-sludge reactor. The membranes are mounted in modules (sometimes called cassettes) that can be lowered into the bioreactor. The modules are comprised of the membranes, support structure for the membranes, feed inlet and outlet connections, and an overall support structure. The membranes are typically subjected to a vacuum (less than 50 kPa) that draws the water from within the bioreactor through the membrane filter to separate clean water from solids. To clean the exterior of the membranes, compressed air is introduced through a distribution manifold at the base of the membrane module. As the coarse, relatively large air bubbles rise to the surface, scouring of the membrane surface occurs, removing solids build-up and slowing fouling of the membrane. The air also provides some dissolved oxygen, but the aeration rate from the coarse bubbles is typically not sufficient to maintain aerobic conditions of the liquor within the membrane module or biological treatment module. In current MBRs, other aeration methods, mainly fine (relatively small) bubbles are typically used to provide the remainder of the dissolved oxygen required to maintain aerobic conditions.
When used with domestic wastewater, MBR processes could produce effluent of sufficiently high enough quality to meet regulations for discharge to coastal, surface or brackish waterways or to be reclaimed for irrigation where appropriate.
As noted above, current methods for adding dissolved oxygen to wastewater use air or oxygen gas bubbles injected through aerator heads (perforated ceramic, plastic or rubber) that are placed at the bottom of the column of water in the biological treatment basin. Bubbles rise through the water and dissolve incompletely. Therefore, deeper basins are needed to increase the efficiency of dissolution and depending on the gas, a gas recovery mechanism may be required.
The efficiency of bubbles dissolving into water (called alpha factor) is also greatly affected by the solids concentration in the target water. The greater the solids concentration, the lower the efficiency of oxygen use from aeration bubbles. Once the solids concentration nears 3%, the use of bubbling technology for oxygenation becomes impractical because of the low efficiency of dissolution and associated high power requirements. Because of this oxygenation limitation, the use of MBR's with wastewater with solids concentrations of near 3% or greater has not been commercially feasible.
Moreover, in MBRs, filtration performance inevitably decreases with filtration time. This is due to the deposition of soluble and particulate materials onto and into the membrane, attributed to the interactions between activated sludge components and the membrane. This major drawback and process limitation has been under investigation since the early MBRs were developed, and remains one of the most challenging issues facing further MBR development.
There is a need therefore, for a MBR wastewater treatment system that is more cost effective than current systems, requires a smaller footprint, is capable of operating efficiently over a wide range of influent and pollutant loading rates and is capable of treating wastewater with a solids concentration of near 3% or greater.